LCRA Water Supply Operations Basic Concepts

Transcription

1 An Overview of LCRA Water Supply Operations August 2018 Lakeside Irrigation System - River Plant 1 LCRA Water Supply Operations Basic Concepts Two basic sources of surface water supply Run-of-River Water (ROR) - natural flow of the river Stored Water - water supplied from lakes Buchanan and Travis Water supply operations are conducted consistent with all water rights, contracts and legal agreements 2 1

2 LCRA Water Supply Operations Basic Concepts Water rights generally grant the holder access to the natural flow of the river subject to conditions. Location Purpose of use Maximum Annual Quantity Maximum Rate of Diversion Priority Date Special Conditions Many water right holders on the Colorado River also contract with LCRA to purchase Stored Water. 3 LCRA Water Supply Operations Basic Concepts Stored Water is supplied from two storage reservoirs in the Highland Lakes system: Lake Buchanan formed by Buchanan Dam Lake Travis formed by Mansfield Dam Water rights for these two lakes authorize LCRA to impound and divert water, and use the bed and banks of the Colorado River to convey water released from lake storage for use by others. 4 2

3 LCRA Water Supply Operations Basic Concepts The Water Management Plan (WMP) is a special condition on the Buchanan and Travis water rights required by the State. The current version was approved by TCEQ Nov Stored Water is accounted for based on amounts supplied to customers that divert from the lakes or released through Mansfield Dam to meet downstream needs. 5 LCRA Water Supply Operations Objectives LCRA manages the system according to the following guidelines: 1. All demands for water from the Colorado River downstream of lakes Buchanan and Travis should be satisfied to the extent possible by run-ofriver flows of the Colorado River pursuant to run-of-river water rights; 2. Inflows should be passed through lakes Buchanan and Travis to honor downstream senior water rights only when those rights cannot be satisfied by the flow in the Colorado River below the Highland Lakes; 3. Water should be released from lake storage to satisfy downstream contractual demands only to the extent that such contractual demands cannot be satisfied pursuant to run-of-river water rights. 6 3

4 LCRA s Highland Lakes 7 Downstream Needs on the Colorado River Storage Reservoirs (Buchanan and Travis) Municipal Users Downstream Demands (Austin and others) Industrial Users (LCRA, Austin Power Plants and others) Instream Flows (Austin, Bastrop, Columbus, and Wharton) Irrigation Operations (Agricultural & Industrial) Corpus Christi Freshwater Inflows (Matagorda Bay) STP Power Plant 8 Matagorda Bay 4

5 Typical Flow Times from Mansfield Dam 9 Detail - Downstream of Columbus Columbus Gauge Lakeside Irrigation Garwood Irrigation Pierce Ranch Irrigation Wharton Gauge Gulf Coast Irrigation & Arbuckle Reservoir (Plant 2 at Lane City) City of Corpus Christi Gulf Coast Irrigation (Plant 1 at Bay City) Gulf Coast Irrigation (Plant 3 at Bay City) Bay City Gauge South Texas Project 10 5

6 Functional Requirements Mansfield releases are scheduled two days in advance. Delivery times vary with conditions, but are typically about 1 to 7 days depending on location. Scheduling of releases requires - at multiple locations along the lower river for 3 to 9 days in the future daily forecasts of: Location and timing of planned diversions Amounts of ROR and Stored Water available to meet needs 11 Modeling System LCRA uses CADSWES RiverWare to make daily forecasts, schedule water releases at Mansfield Dam and track water use. CADSWES University of Colorado, Center for Advanced Decision Support for Water and Environmental Systems RiverWare software platform funded and used by U.S. Bureau of Reclamation Tennessee Valley Authority U.S. Army Corps of Engineers others 12 6

7 LCRA Daily River Operations Model (DROM) LCRA DROM includes two RiverWare models that support daily forecasts, daily schedules to release water at Mansfield Dam and water use accounting. Routing Model - Estimates the run-of-river (ROR) available to satisfy demands of water right holders Release Model - Determines the amount of ROR and Stored Water that must be released at Mansfield Dam to satisfy all downstream demands, consistent with all water rights and agreements. 13 ROUTING MODEL Considers recent historical data for releases from Mansfield Dam diversions and return flows gauged inflows from tributaries Provides estimates of: un-gauged (run-of-river) gains and losses of flow flow attributable to run-of-river and stored water The routing model operates on an hourly time-step. 14 7

8 ROUTING MODEL - Simplified Schematic Lake Travis Mansfield Dam Municipal diversions from Lake Travis Austin CoA WWTP return flows Bastrop Streamflow Gauge Major Diversion Various Tributary Stream Gauges Garwood Irrigation Pierce Ranch Irrigation Fayette Power Project Columbus Lakeside Irrigation Wharton Gulf Coast Plant 2 Corpus Christi Municipal Gulf Coast Plant 3 South Texas Project Gulf Coast Plant 1 Bay City Matagorda 15 Bay What about un-gauged gains or losses between gauges? Lake Travis Mansfield Dam Municipal diversions from Lake Travis Austin CoA WWTP return flows Bastrop Streamflow Gauge Major Diversion Buckners Creek Stream Gauge Garwood Irrigation Pierce Ranch Irrigation Fayette Power Project Columbus Lakeside Irrigation Wharton Gulf Coast Plant 2 Corpus Christi Municipal Gulf Coast Plant 3 South Texas Project Gulf Coast Plant 1 Bay City Matagorda 16 Bay 8

9 To estimate un-gauged gains or losses between gauges, Hourly Average Discharge (cubic feet per second) we start with the observed hydrograph at the upstream gauge. Observed Hydrograph at Bastrop Time (Hours) 17 The Routing Model routes the observed upstream hydrograph to the downstream location. Hourly Average Discharge (cubic feet per second) Routing simulates the physical effects of flow lag time and channel storage. The hydrograph tends to flatten as it moves down the river channel. Flow Lag Time Observed Hydrograph at Bastrop Bastrop Hydrograph Routed to Columbus Time (Hours) 18 9

10 Hourly Average Discharge (cubic feet per second) The Routing Model compares the routed and observed hydrographs at the downstream location. Observed Hydrograph at Bastrop Difference between Routed and Observed Hydrographs at Columbus Observed Hydrograph at Columbus Bastrop Hydrograph Routed to Columbus Time (Hours) 19 The difference is adjusted for known inflows and outflows to estimate the un-gauged portion of inflows. For example: Bastrop Buckners Creek (gauged tributary) Fayette Power Project (metered diversion) Un-gauged Inflow between Bastrop and Columbus Un-gauged (Lateral) Inflow between Bastrop and Columbus = Observed Flow at Columbus Routed Flow at Columbus - Buckners Creek (Tributary gage) + Fayette Power Project Diversion Columbus 20 10

11 RiverWare Simulation View - Bastrop to Columbus The RiverWare model is more complex than shown on the simplified schematic map on the previous slide. Bastrop Gage Buckners Creek Gage (tributary) Fayette Power Project diversions Columbus Gage 21 ROUTING MODEL USE OF OUTPUTS The routing model provides estimates of historic mean daily values, based on recent history, for: un-gauged (run-of-river) gains and losses of flow allocation of flows to run-of-river and stored water When weather conditions are expected to remain stable, i.e., consistently wet or consistently dry, recent history provides a good basis for estimating flows in the near future. When weather conditions are unstable, the modeler must exercise independent judgment to forecast future flows

12 RELEASE MODEL Considers expected demands for senior water right holders, stored water contract customers and environmental flow requirements. Considers forecasted ROR water entering the river downstream of Mansfield Dam, (downstream ROR), available to meet demands of ROR water right holders. Considers previous day gauged inflows to the Highland Lakes (upstream ROR) available for pass through to downstream water rights senior to the Highland Lakes. 23 RELEASE MODEL Considers legal framework of prior appropriation among water rights contractual obligations between LCRA and water users environmental criteria Calculates the minimum discharges from Mansfield Dam for water supply purposes based on priority allocation among all water rights, contracts, and other requirements. Forecasts expected flows downstream of Mansfield Dam. The release model operates on a daily time-step

13 Priority Dates for Major Water Rights on the lower part of Colorado River Rank Effective Priority Date* Water Right holder 1 November 1, 1900 Garwood Irrigation Division 2 November 2, 1900 City of Corpus Christi (available at Garwood diversion point) 3 June 30, 1913 City of Austin municipal use (from L. Austin for Davis WTP and Ullrich WTP ) 4 July 1, 1913 Gulf Coast Irrigation Division {unsubordinated date Dec. 1, 1900} 5 July 2, 1913 Lakeside Irrigation Division (senior portion) {unsubordinated date Jan. 4, 1901} 6 July 3, 1913 Pierce Ranch {unsubordinated date Sept. 1, 1907} 7 July 4, 1913 Lakeside Irrigation Division (junior portion) {unsubordinateddate Sept. 2, 1907} 8 November 12, 1913 Bastrop Energy 9 June 27, 1914 Austin Energy at Fayette Power Project (available at Lady Bird Lake) 10 June 27, 1914 City of Austin additional municipal for L. Austin and LBL 11 March 29, 1926 LCRA Highland Lakes (impoundment) 12 March 7, 1938 LCRA Highland Lakes (diversion) 13 August 20, 1945 Austin Energy at Decker Power 14 August 20, 1945 City of Austin additional municipal from Colorado River d/s of LBL 15 June 10, 1974 South Texas Project 16 November 1, 1987 Lakeside Irrigation Division (1987 portion) 17 November 1, 1987 Gulf Coast Irrigation Division (1987 portion) 18 September 23, 1997 City of Corpus Christi (flow d/s of Garwood diversion point) 19 January 28, 1999 Austin Energy at Fayette Power Project (flow d/s of Lady Bird Lake) *As modeled in DROM. Note, for example, that by separate agreement, some LCRA water rights have been subordinated to the City of Austin municipal water right. 25 Llano River at Llano Colorado River near San Saba RELEASE MODEL Simplified Schematic Buchanan Dam (Lake Buchanan) Sandy Creek near Kingsland Pedernales River near Johnson City Austin Gage LCRA s Highland Lakes (from L. Buchanan to L. Travis) Mansfield Dam (Lake Travis) City of Austin diversions CoA WWTP return flows Gaged Streamflow Location Major Diversion Location Bastrop Gage Buckners Creek Gage Fayette Power Project Columbus Gage Garwood Irrigation Lakeside Irrigation Pierce Ranch Irrigation Wharton Gage City of Corpus Christi diversion Gulf Coast diversion at Lane City Gulf Coast diversion at Bay City Bay City Gage South Texas Project Matagorda 26 Bay 13

14 RELEASE MODEL Allocation of ROR Uses accounting features of RiverWare to allocate ROR water to meet expected demands for each major diverter that holds a water right Allocation is performed in two rounds: Round 1 - considers the ROR water originating downstream of Mansfield Dam (also called Below Travis Run-of-River ) to optimize use of this supply source. Round 2 - considers the ROR water originating upstream of Mansfield Dam (also called Above Travis Run-of- River and Inflows into the Highland Lakes ). 27 RELEASE MODEL Above Travis Run-of-River Colorado River near San Saba Llano River at Llano Sandy Creek near Kingsland Buchanan Dam (Lake Buchanan) LCRA s Highland Lakes (from L. Buchanan to L. Travis) Mansfield Dam (Lake Travis) Gaged Streamflow Location Pedernales River near Johnson City Above Travis Run-of-River or Inflows into the Highland Lakes* *Daily inflow into the Highland Lakes is calculated as the sum of MDD at: x DAR Colorado/San Saba Llano/Llano 1.0 Sandy Creek/Kingsland Pedernales/Johnson City 2.03 * Inflows are calculated by applying Drainage Area Ratio (DAR) method to the mean daily discharge (MDD) at four inflows gauges located upstream of the Highland Lakes. Matagorda 28 Bay 14

15 RELEASE MODEL Releases of ROR and Stored Water Allocates Above Travis Run-of-River water to any downstream water rights senior to lakes Buchanan and Travis that have not been met with Below Travis Run-of- River water. Allocates Stored Water to any remaining portion of orders for diverters with LCRA water supply contracts, that have not been met or cannot be met with ROR water. Releases of stored water to meet Water Management Plan environmental flow criteria for freshwater inflows to Matagorda Bay are determined each month and scheduled when required. 29 RELEASE MODEL Instream Flow Releases Releases of the Above Travis Run-of-River and stored water needed to meet downstream needs of senior waters, water supply contracts and freshwater inflow needs are simulated. Simulated flows are compared to Water Management Plan criteria for instream flows at Austin, Bastrop, Columbus, and Wharton. Additional releases of Above Travis Run-of-River or stored water may be required to meet instream flow criteria, depending on specific conditions of the Water Management Plan

16 RELEASE MODEL Scheduled Releases Recommends the amount of water to be scheduled for release from Mansfield Dam. The release is the sum of Pass-through release of Above Travis Run-of-River Release of Stored Water from lakes Buchanan and Travis Forecasts flow at locations downstream of Mansfield Dam. 31 RiverWare Example of Simulation View The RiverWare model is more complex than shown on the simplified schematic maps

17 RiverWare Example of Accounting View The RiverWare model is more complex than shown on the simplified schematic maps. This is same portion of the river as previous slide, but this shows the accounting features. Each color is a different water supply, such as Below Travis Run-of- River